A video monitor receives control signals including vertical and horizontal synchronising signals, and uses these signals to control the display of a video image. In a conventional video monitor a horizontal position control circuit receives the horizontal synchronising signal and uses the horizontal synchronising signal to control the horizontal scanning across the display. In addition, the horizontal position control circuit provides a means of adjusting the horizontal position of the video image.
To control the horizontal scanning, the horizontal position control circuit generates a horizontal drive output signal to a deflection circuit when a horizontal synchronising pulse occurs. However, a deflection circuit has an inherent propagation delay and the propagation delay varies between different deflection circuits. The propagation delay causes a delay between the time the horizontal synchronising pulse occurs and the time the horizontal drive output signal is generated. To overcome the difficulty caused by the propagation delay of the deflection circuit, a known horizontal position control circuit advances the generation of the horizontal drive output to the deflection circuit by a time equal to the propagation delay of the deflection circuit. The known horizontal position control circuit does this by generating a reference ramp signal having a ramp with a duration that is high enough to cover a range of propagation delays, and then using the ramp to generate the horizontal drive output signal.
Conventionally, the reference ramp signal is also used in the horizontal control circuit to provide a range of horizontal position adjustment. This method of dual use of the ramp of the single reference ramp signal is used in the MC13081 integrated circuit manufactured by Motorola Inc. In order to accommodate the requirements of wide propagation delay compensation and horizontal position adjustment, the duration of the ramp of the reference ramp signal can be increased causing the ramp of the single reference signal to have a more gradual slope. However, with a more gradual slope the noise in the horizontal control circuit increases, which results in jitter of the video image on the display.
Hence, there is a need for a horizontal control circuit that can provide a wide range of propagation delay compensation and a range of horizontal position adjustment, without producing a large amount of jitter of the video image.